In power generating plants, heat transfer between the steam and water is accomplished by means of feedwater heaters. Such heaters typically operate under high pressures on the order of 1,000 to 5,000 PSI, and must be constructed to withstand such pressures and to permit access to the interior for inspection, cleaning and the like. Feedwater heaters generally comprise a cylindrical casing, a removable closure assembly, and a seal therebetween. The closure assembly is usually designed so that internal pressurization of the feedwater heater facilitates and maintains a proper seal with the casing. Metallic gaskets of various types have been used for this purpose, however, the designs of the closure assemblies of the prior art have tended to cause extreme deformation of the gaskets, shortening their service lives and making removal of the closure assemblies a difficult, time consuming task. Specialized equipment and techniques have been required to remove the closure assembly from the casing in some circumstances. It is thus desirable to design the closure assembly and seal to provide effective sealing when the feedwater heater is under pressurization, while enabling complete disassembly after operation without the difficulties which have characterized the prior art.
U.S. Pat. No. 4,239,124 shows a fluid-tight closure apparatus representative of one prior art approach to this problem. In this device, the inside end of the closure wall is formed to receive a circular pressurizing ring which contacts a surrounding seal and packing ring to effect sealing engagement with the inside surface of the casing. This approach, however, requires specially formed parts and is still subject to some of the difficulties of the prior art. Since all of the compression force is transmitted through the gasket, a relatively large bearing area is required. This type of gasket is usually of one-piece construction, which is expensive and not readily available. Further, after compression, this gasket also tends to wedge between the adjacent parts making separation and removal difficult.